This invention relates to the art of a method and apparatus for thermal recording using a thermal head.
Thermal recording materials comprising a thermal recording layer on a substrate of a film or the like, which are hereunder referred to as thermal materials, are commonly used to record images produced in diagnosis by ultrasonic scanning.
This recording method, also referred to as thermal recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence in recent years, the use of the thermal recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal recording involves the use of a thermal head having a glaze in which heat-generating elements for heating the thermal recording layer of a thermal material to record an image are arranged in one direction (main scanning direction) and, with the glaze a little pressed against the thermal material (thermal recording layer), the thermal material is relatively moved in the auxiliary scanning direction perpendicular to the main scanning direction, and the heat-generating elements of the respective pixels on the glaze are heated by energy application in accordance with image data to be recorded which were supplied from an image data supply source such as MRI in order to heat the thermal recording layer of the thermal material, thereby accomplishing image reproduction.
The glaze of the thermal head has a protective layer formed on the surface in order to protect the heat-generating elements for heating thermal materials, the associated electrodes and the like. It is this protective layer that contacts the thermal material during thermal recording and the heat-generating elements heat the thermal material through this protective layer so as to perform thermal recording.
The protective layer is usually made of wear-resistant ceramics; however, during thermal recording, the surface of the protective layer is heated and kept in sliding contact with the thermal material, so it will gradually wear and deteriorate upon repeated recording.
If the wear of the protective layer progresses, uneven densities will occur in the thermal image or the desired protective strength can not be maintained and, hence, the ability of the layer to protect the heat-generating elements is impaired to such an extent that the intended image recording is no longer possible (the head has lost its function). Particularly in the application such as the aforementioned medical use which require variable-contrast images of high quality, the trend is toward ensuring the desired high image quality by increasing the settings of recording temperature (energy applied) and of the pressure at which the thermal head is urged against the thermal material, and the protective layer on the thermal head is more prone to deterioration by wear, which largely affects the operational reliability and life of the thermal head.
With a view to preventing the wear of the protective layer on the thermal head, various ideas have been proposed and implemented and they include improvements on the constituent material of the protective layer, its layer arrangement and the recording conditions, as well as methods directed to the formation of a coating layer on the surface of the protective layer.
Among these proposals, the method of forming a coating layer on the surface of the protective layer has been the subject of intensive research and development efforts since it is capable of not only preventing the deterioration by wear of the protective layer but also refilling the pinholes formed in its surface, thereby ensuring that the operational reliability of the thermal head can be greatly improved in comparatively simple ways.
To mention a few examples of the proposals so far made in this direction, they include: Unexamined Published Japanese Patent Application (Kokai) No. 193853/1988 which discloses a thermal head that has an electrically conductive paint applied to the surface of the protective layer to form a coating layer which not only provides improved reliability but also prevents static buildup which would otherwise cause the failure of the heat-generating elements; Kokai No. 251255/1988 which discloses a thermal head that has an anaerobic resin coating formed on the surface of the protective layer to ensure that the pinholes are closed while providing improved reliability; Kokai No. 232762/1992 which discloses a thermal head designed to close the pinholes in the surface of the protective layer while preventing its corrosion by employing a coating agent that has specified values of viscosity, surface tension, softening temperature, thermal decomposition temperature and withstand voltage; and Kokai No. 84948/1993 which discloses a thermal head also designed to close the pinholes in the surface of the protective layer while preventing its corrosion by forming a coating layer made of an alkoxide glass having specified values of viscosity and surface tension.
With these thermal heads having a coating layer formed on the surface of the protective layer, the coating layer can initially attain the intended object by effectively preventing the corrosion and wear so as to ensure the reliability of the thermal heads. On the other hand, these thermal heads heat the thermal recording layer through the coating layer which is kept in sliding contact with the thermal material and, hence, the coating layer will eventually wear upon repeated recording. As a result, in extensively worn areas of the coating layer or in areas where the coating layer has worn out completely, the protective layer is either worn or corroded as in the case of the thermal head having no such coating layer and the reliability and life of the thermal head are inevitably reduced.